1. Field of the Invention
The present invention relates to a sheet stack loader and a sheet loading method used in a sheet post-processing apparatus installed on the later stage of a copying machine or others.
2. Description of the Related Art
Various sheet post-processing apparatuses for performing a process of folding a sheet bundle formed by stitching, making a hole in, and saddle-stitching image-formed sheets by an image forming apparatus such as a copying machine, a printer, or a facsimile are proposed. Generally, there is a sheet post-processing apparatus available having, for example, two ordinary sheet receiving trays for loading ordinary sheets, stitched sheet bundles, and sorted sheet bundles on the upper part of the side of the housing and furthermore separately having an exclusive folded sheet receiving tray for loading folded sheets obtained by saddle-stitching a plurality of sheets at the center using a stapler and furthermore folding them at the center using folding rollers and a folding plate.
In the sheet post-processing apparatus having the two kinds of receiving trays aforementioned, the folded sheet receiving tray is installed under the ordinary sheet receiving tray. Therefore, the folded sheet loading method uses a system to transport folded sheets through the transport port from the folding and stitching side thereof and sequentially load them straight on the top of the folded sheets loaded already.
However, by this method, when many folded sheets are loaded and the height of folded sheets reaches close to the transport port, a jam of rushing of folded sheets transported later into the folded sheets loaded already from the side not folded and stitched is caused, thus normal transporting and loading of folded sheets are obstructed. To avoid it, it is necessary to restrict the loading quantity or to install the transport port of folded sheets at a higher position. However, if there is an excessive difference of elevation between the transport port and the receiving tray, loading of folded sheets becomes unstable. Or, the vertical driving range of the ordinary sheet receiving tray is restricted, thus it results in restriction on the loading quantity of the ordinary sheet receiving tray.
For this problem, as disclosed in Japanese Patent Application Publication No. 2002-104712, a sheet post-processing apparatus with no exclusive folded sheet receiving tray installed to share the ordinary sheet receiving tray and folded sheet receiving tray is proposed. This post-processing apparatus has two upper and lower ordinary sheet receiving trays driving on the side of the housing of the apparatus, and the folded sheet transport port is installed immediately under the ordinary sheet transport port, thus the trays can be shared.
However, in the conventional sheet post-processing apparatus, to install the folded sheet transport port immediately under the ordinary sheet transport port, the conveying path in the apparatus must be made longer, resulting in an increase in cost. Further, the receiving trays are shared, so that under the condition that ordinary print and folding print are job-reserved alternately, the receiving trays are moved to the respective transport ports, and then the sheets are transported, so that the switching time is required inefficiently. Furthermore, under the condition that the reservation is continued such as ordinary printing, taking a plurality of copies using the sorting process, and taking a plurality of printings by the folding process, in the receiving trays, folded sheets and ordinary sheets are overlaid, and the loading balance gets worse or for example, when intended to transport sheets using the sorting process to the ordinary sheet receiving tray, sheets ordinarily printed and sheets using the sorting process are mixed on the receiving tray and the sorting process requires much labor and time.